'It used to be that 100% of my audience was in the US, but now it’s in Brazil and Turkey': Susan Miller. Photograph: Philip Toledano for the Observer

Susan Miller has important news for us. "April's so scary that I'm giving classes on it," she says, tracing a series of points on her impossibly complicated astrology chart. "Look, we have a perfect square on 15 April – 15 April! You've got Jupiter at 12, and Uranus at 13, and Pluto at 13, and Mars at 16 – but wait! It's going to get a little bit worse." She furrows her brow while she studies the chart. "Look at 29 April!" I look. "Some people feel the stock market is…" She pauses for such a long beat that I offer to complete her sentence: "…going to crash?" She shakes her head. "This is even worse – we've not had this since the American Revolution."

One thing you quickly appreciate about Susan Miller, America's most popular astrologer, is how relentlessly upbeat she sounds, even (and perhaps especially) when delivering earth-shattering news. "We'll start at the beginning," she says with a bright rah-rah jolliness as I settle into a seat at her local Dunkin' Donuts on Manhattan's Upper West Side for our first meeting. "I was born with a terrible birth defect, and doctors thought I was making it up, that I was very clever and found a way to stay out of school for eight weeks at a time."

This story – like all of the stories – is long and winding, but the upshot is that Miller spent great chunks of her childhood bedridden and in pain. It's hard to imagine anyone wringing anything positive from this tale of woe, but Miller does. "You know how people say: 'I want to get out of hospital so I can live my life'? Well, you're living your life all the time – every experience you have adds to who you are, and it gives you insight."

Miller is garrulous, warm and utterly unpretentious. She is also, according to her many, many fans, uncannily accurate. This first meeting occurs during the US government shutdown over the budget last October, and Miller is riding high after forecasting a diabolical month. Her reading for Leo, the birth sign of President Obama, began: "You may have already heard the rumours – October is not due to be an easy month in any which way" and continued: "The new moon may trigger contract negotiations, but talks are likely to hit snags. Don't try to rush things to closure. Delays will benefit you." The government impasse ground on before being finally broken in Obama's favour.

Miller, a grocer's daughter from Manhattan, learned astrology in her teens at the knee of her reluctant mother, who had to home-school her for much of the time. Forced to wear a leg brace at 14, she wanted to see into the future, wanted to know if she'd ever be able to wear heels again. She began doing readings for relatives, then friends and acquaintances. "I knew who I was because of astrology," she says. "All my friends were, like: 'Who am I? Where am I going?' I never had that."

Today millions of people look to Miller to tell them who they are, and where they are going – 6.5 million online every month, and rising. "It used to be that 100% of my audience was in the US, but now it's between 42 and 47%," she says. "Now it's Brazil and Turkey who love astrology and who love the internet." Does she notice regional differences in what people want to know? "No – the more I study it the more I realise that the CEO and the janitor have the same questions: is my wife going to come back to me? Can we have children? Is my work going to be steady?"

In her home city, the janitor and the CEO are joined by the fashion designer and the magazine editor, for whom Miller's extended monthly forecasts, available free of charge on her site, astrologyzone.com, are catnip. Some of that is down to Miller's association with publications such as InStyle and Elle, but it's also a reflection of the industry's incestuous nature. "Part of it is that we're just such blabbermouths," says Mickey Boardman, the editorial director of New York's downtown bible, Paper, who only has to mention his friendship with Miller to find his stock precipitously rising. "Suddenly they're emailing me every day wanting to have dinner with her – and I don't blame them," he says.

"She's just part of the fashion world now," says Miller's oldest daughter, Chrissie Miller, who consults as a creative director for modish fashion brands such as Warby Parker and frequently finds herself upstaged by her mum's fame. "I had someone cry when they found out," she says. Chrissie had just got back from Art Basel in Miami and had run into Glenn O'Brien – GQ magazine's "style guy", whose association with fashion and art goes back to his time working with Andy Warhol at Interview magazine. "He just loves my mom, and he was like: 'Please thank her for me. I bought a car right after Mercury, and I signed a lease.'" (O'Brien later confirmed this story, adding that the fashion and art worlds had always expressed a natural affinity for astrology. "Creative people are natural pagans," he said. "It's the only way you get to talk about Venus and Mercury and Jupiter.")

After finding out that Chrissie's mother was the Susan Miller, fashion designer Cynthia Rowley was similarly effusive. "I almost died, I was so starstruck," she admits. "I've been a fan for many years." Rowley credits Miller for anticipating both her husband's proposal of marriage and the birth of her first child. "I don't know that much about astrology, but on the first of every month I wait until Susan Miller posts," she says. "I'm completely faithful to her and have such unbelievable in-synch moments both when I follow her advice and when I don't – so now I only follow her advice."

For Bevy Smith, a host on Bravo TV's style show Fashion Queens, Miller's horoscopes are akin to familiar friends dispensing worldly advice. "I've had two contracts for TV shows that I have not signed because of Susan," she confesses. "If they want you, they'll wait, so I'd rather err on the side of Susan."

Friends in high places: with her daughter Chrissie Miller, Jerry Hall and Georgia May Jagger. Photograph: Jamie Mccarthy/Getty Images for Sunglass Hut

Boardman, too, does not countenance any criticism, warning: "I hope this is not some kind of evil exposé about how all astrology is lying, because all my fashion people and I will hunt you down and kill you." He met Miller after tweeting her, and the two quickly bonded. "You expect someone in a Stevie Nicks outfit, with lots of cats and crystal balls and damaged hair, and she's actually very chic, pretty, put together," he says. Perhaps because of that she has become a popular attraction at in-store fashion events, where she dispenses such pearls as "Accessories are going to explode in the second half, because Jupiter's in Gemini" and "With Uranus in Aries, we're moving away from the sparkles", both at a Juicy Couture launch in 2012.

The heiress and socialite Gloria Vanderbilt, who contributes her own watercolours to illustrate Miller's calendars, describes her simply as "the guru of astrology". In return Miller recalls a recent conversation with Vanderbilt. "She is 89 and asked me: 'Do you see romance in my life? I'd love to meet a man.' I love this! I love it. I want to be that way."

To the sceptic, astrology is a pseudo-science. I emailed the writer and famous window dresser Simon Doonan to see if he was familiar with Miller, and received a curt two-line email: "Do not really follow astrology. My grandpa was a professional astrologer who got up one day and shot himself."

Future fit for a king: Prince George in the arms of his parents, Kate and William. Photograph: Kirsty Wigglesworth/AP

Miller is sensitive to critics, and acknowledges that the profession hasn't always put its best foot forward. A brief deal with Barnes & Noble ended unhappily after the chain tried to sex up a book of Miller's that it was publishing. "They were putting palms in it and things," Miller says crossly. "I was like: 'What's this? I'm not a palm reader!'" She adds that people don't always appreciate that a horoscope is a two-way street. "I can see when the cycle will offer opportunity, but it's not about setting your watch and waiting for someone to knock on the door," she says. "You have to contribute the energy."

Has she ever got it wrong? She giggles and nods. In 2000 when seven planets were aligned she was convinced that catastrophe was nigh. "Here were seven planets in Taurus – an earth sign – and Uranus was square in Aquarius, and I'm thinking: 'What could possibly happen?' I thought there would be an earthquake, and boy was I wrong! Uranus, being the little bad boy, was the perpetrator – it was the day of the Love Bug that went around the world and ruined all those computers." In the scheme of things this doesn't seem such a grave misstep. Something bad did happen. "Yes, but I was looking at Taurus, and all along I should have been looking at Uranus," she says, as if this should have been obvious to any five-year-old with an astrological chart in their hands.

I'm still absorbing this when Miller sweeps on to another subject. "I have to tell you something very interesting about the royal family," she says, looking me in the eye. "Prince George was born a hair's-breadth away from Leo, but he's still Cancer – 29 degrees and 59 minutes. But I'm glad he's Cancer, because his father, Prince William, is Cancer, his mother is Capricorn. That's good. Water and earth make flowers. Her moon is in Cancer. Her father, the baby's grandfather, is Cancer. Uncle Harry is a Virgo. Pippa is born the same week as Harry. Virgo. So, earth and water is good. Great grandmother, Queen Elizabeth, is a Taurus, and grandfather Charles is a Scorpio. Earth and water again. All good. This baby fits in so perfectly it's insane!"

It's Miller's level of detail and depth that keeps people coming back. "Her forecasts are like the War and Peace of horoscopes," says Bevy Smith, who admits to waiting on tenterhooks for Miller's monthly updates. The allusion to Tolstoy's doorstop of a book is deliberate: Miller's forecasts run to 48,000 words, an anomaly in the age of the tweet, and a point of principle for Miller, who refuses all overtures to slim them down.

She remembers scoring her first online gig in 1995 – way back in the primordial swamp of online media – for Time Warner's short-lived website Pathfinder. "They said: 'We want something short for women every day.' And so I said: 'No, it should be long for men and women every month, not short for women every day.' They said: 'How long?' At the time I said 17,000 words. They said: 'Won't you get tired?' I replied: 'Who tired? Me tired? No! No, no, no, no, no. I don't get tired.'"

She claims to work around 20 hours a day, and is fond of quoting her mother, who told her: "When you grow up and you start working, here's what you have to remember – produce or get out." (Her mother died last year at the age of 93, and Miller has not held back from sharing her grief with her fans.)

Miller has the brisk instincts of a businesswoman who understands what it takes to build a brand. She remembers showing her father how to market his store by composing signs – "We finally have arugula! Come in!" – to display outside. "People said: 'I don't know what that is but I want it,'" she recalls. "My father was so worried about the big chains, but I said: 'We have personal service, Daddy, don't worry.'"

A consistent feature of Miller's path to astrological fame is her easy ability to ingratiate herself with decision-makers. That hand-written sign for arugula was a harbinger of things to come. She appears to have won everlasting support at Apple by sending them a prospectus in a red velvet box inspired by Snow White, a childhood favourite. "I thought: 'Velvet box – everything I send out will be in a velvet box, so to speak', you know? And it is true to this day." If it's difficult to imagine Steve Jobs displaying his velvet box on his desk, Miller's savvy exercise in self-marketing opened doors at Apple HQ. "They gave me Apple's birthday, and they gave me Steve's – you know, 24 February 1955, that magical year when Bill Gates was also born," she says, wide-eyed with wonderment. "Jupiter was conjunct with Uranus – oh, '55 was magical for invention." Armed with the data she needed, Miller apparently produced a forecast for Apple that corresponded with its forthcoming heavily guarded releases. "They were flabbergasted," she recalls. "They ran a story on their site: 'The astrologer who believes in Apple', and kept it up under Hot News for 10 years."

Miller rarely gives private readings, but for a later interview she has asked me to call my mother and confirm my time of birth. We are in a restaurant for dinner, and while we wait for the food she pulls out her computer and taps in my details. "Do you keep moving your furniture around?" I do not. Miller tries again. "Are you always throwing parties or having people over, or people to sleep over?" Not always, but sometimes. "Do you have Grandmother's china there and everything?" As a matter of fact I do, though "everything" seems like a stretch. She nods knowingly. "It's getting more valuable by the minute, probably growing faster than Apple stock [I checked, and it's not – but then neither is Apple stock growing right now]. Oh by the way, the weakest part of your body is your ankles, so if you're skateboarding or something you really have to pay attention. The other part is your head, which includes teeth, and Saturn is in Aries. Saturn could give you headaches or teeth pain."

Miller told me some very nice things about my future, but I wasn't feeling especially persuaded until I began writing this piece and came down with a chronic toothache. Dosed up on painkillers, I made my way to a dentist, who carried out an immediate root canal. If I hadn't been checking my notes for this story I might have forgotten Miller's observation about my teeth. So I downloaded Miller's app and I've found myself checking it casually each morning. Unexpectedly, I find myself spotting observations that correspond with my life. At the start of the month, my forecast warned of yet more tooth pain to come, and lo, my dentist confirms that another root canal is on the cards. Maybe it is just a coincidence, or English teeth – my ankles feel fine, after all – but, like Bevy Smith, I've decided that I'd rather err on the side of Susan.

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